1. Field of the Invention
The present invention relates to a multi-color image forming apparatus using multiple beams in digital copying machines, laser beam printers, and the like.
2. Description of the Related Art
Conventionally, as a multi-color image forming apparatus, a device to modulate two laser beams in accordance with the image data and irradiate to different portions in the circumferential direction of the photosensitive drum, repeat charging and developing with respective positions in between, and form a two-color image in one rotation of the photosensitive drum is well known.
In this type of a color image forming apparatus, developing device containing color toners other than black such as red or blue are generally located at upstream side of the developing device containing a black toner with respect of the rotating direction of the photosensitive drum. This is because the black toner, when colors are mixed, exhibits a marked color contamination. In this type of color-image forming apparatus, the image data to output laser beams to be irradiated downstream said is delayed with respect to that upstream side considering the rotating time of the photosensitive drum to each irradiation position in order to align the multiple colors of the image formed on the photosensitive drum.
In this type of color-image forming apparatus, in the case where the laser source as an exposing means which exposes the photosensitive drum and/or developing devices which feeds toners to the latent image on the photosensitive drum formed by exposing become inoperable, the apparatus becomes incapable for forming images. In such event, a specialist engineer must be consulted to repair the inoperable irradiator and/or developing devices.
However, even in the case where a specialist engineer is consulted, the apparatus with inoperable irradiator and/or developing devices cannot form images until the repairing is completed, and the image-forming apparatus must be shut down. Consequently, frequent occurrence of the inoperable condition as described above generates a problem of marked decrease in the image-forming efficiency.